1. Field of the Invention
The present invention relates to a method of processing image data, and more particularly to a method of processing image data to detect and recognize a defect produced or foreign matter mixed in a process of fabricating a printed circuit board, a semiconductor wafer, or the like.
2. Description of the Related Art
Defects produced or foreign matter mixed in a process of fabricating a printed circuit board, a semiconductor wafer, or the like are responsible for defective final products. Therefore, such defects and foreign matter should be detected as soon as they are produced or mixed.
There have heretofore been many various image processing methods of detecting, recognizing, or judging defects or foreign matter contained in a printed circuit board or a semiconductor wafer based on a photograph thereof or an image thereof which has been captured by an SEM (Scanning Electron Microscope). Most of these image processing methods process image data digitally with a computer.
Conventional image processing for the detection of defects has mainly relied upon the technique of pattern matching. According to the conventional image processing, an image of an object to be inspected is compared with an image of a printed circuit board or a semiconductor wafer that is free of any defects (hereinafter referred to as a "golden device"). Specifically, the image comparison means the generation of a differential image between two pixel values for each pixel pair of the compared images. If the object being inspected has turned out to be free of any defects based on the differential image, then since the image of the object is exactly the same as the image of the golden device, the differential image is an entirely flat image whose pixel values are "0" over the whole image. If the object being inspected has a defect or contains foreign matter, then pixels whose values are other than "0" collectively appear at the defect or foreign matter, and look like a particle. Those pixels whose values are other than "0" are gathered into a cluster (particle-like point), and the defect or foreign matter is detected by determining the size and center of the cluster.
The above defect detecting process, which has already been established in the art, is disadvantageous in that it is necessary to prepare the image of a golden device in advance. Generating the image of a golden device, i.e., the image of a defect-free device, requires that a defect-free sample be found first. Such a defect-free sample needs to be discovered carefully by the human eye. Since patterns for semiconductor wafers are available in a wide range and are being modified frequently due to recent trends toward the fabrication of many different types of semiconductor wafers in small quantities, it is necessary to generate golden device images respectively for all the different types and design modifications. The process of generating those golden device images is highly tedious and time-consuming. Furthermore, before the image of a golden device is actually compared with the image of an object to be inspected, the images need to be aligned accurately with each other. As a result, a considerable amount of pre-inspection processing which includes the generation of the images of golden devices and the accurate alignment of images to be compared has been required prior to the actual inspection process.